Present-day telephones must be placed in an on-hook status to receive an incoming call. This means that the telephone line must be disconnected from the voice network of the telephone system in order that an incoming call may activate the ringer circuit. This on-hook status may be provided by physical placement of a telephone handset on a cradle or a specified surface, or by other mechanical or electronic means. A call waiting system can inform the local party that a second remote party is calling. Alternatively, in telephone lines having the call waiting feature, electronic means may be used to detect a second incoming call to the telephone line, and such electronic means may be used to disconnect the local party from the first caller.
If the local telephone user inadvertently neglects to restore the local telephone to on-hook status, various telephone networks supply a distinctive set of tones (the "howler" signal) to alert the user after the phone has remained off-hook without being connected to the voice network for a given period of time. Such a signal is ineffective for alerting the user, however, if the user is not near the telephone or in general does not hear the signal.
U.S. Pat. No. 4,922,529 issued to D. J. Kiel describes an automatic hangup apparatus that comprises inadvertent-off-hook signal detecting means, output signal generator means, and output transducer means for producing one of two sensible response states according to whether an inadvertent-off-hook status is detected. Kiel further describes the case in which the detecting means comprises a howler signal detector. Kiel also mentions the case in which the determination of inadvertent-off-hook status is based on either (1) the existence of a dial tone for an excessive period of time, or (2) the combination of off-hook telephone status with line silence for an excessive period of time, or (3) the combination of off-hook status with motionlessness of the telephone handset.
Some drawbacks of determination methods (1), (2), and (3) are noted by Kiel. An additional drawback of method (2) is that there may be background sound (such as radio, TV, conversation) that prevents the inadvertent-off-hook status from being correctly detected. A drawback of Kiel's howler detection method is that not all telephone networks may provide "howler" signals, and that the particular form of the signals may differ among those telephone networks that do provide such signals. In the latter case the detecting apparatus may need to be configured differently for use on each such network.
One way in principle to overcome the last-stated drawback would be to use automatic means for recording the howler-type signal that is produced by the network on which the telephone is to be used, perform appropriate signal processing on the analog or digital recorded signal, and use the results of such signal processing to determine a set of criteria to be used for detecting the presence of substantially the same signal in the future. This approach may be costly, inconvenient, and prone to error in the processing of signals possibly accompanied by noise or distortions, and it is ineffective when no howler-type signal is provided by the telephone network.
Consequently, it is desirable to provide a means of detecting inadvertent-off-hook status of a telephone without relying on the provision of a howler signal by the telephone network, and without relying on the other methods stated above.
U.S. Pat. No. 4,998,271 issued to Tortola et al also describes detection of a howler signal. U.S. Pat. No. 4,156,799 issued to Cave describes detection of the hangup click of a remote telephone. Other references that relate to the off-hook status of a telephone are U.S. Pat. Nos. 3,038,965 (Civitano), 4,788,712 (Umemoto et al), 5,142,572 (Hopkins), and 3,150,237 (Baldik et al).